Re: [PATCH v29 23/32] x86/cet/shstk: Add user-mode shadow stack support

Yu, Yu-cheng
2021-08-27T18:10:45Z

On 8/26/2021 9:25 AM, Borislav Petkov wrote:
> On Fri, Aug 20, 2021 at 11:11:52AM -0700, Yu-cheng Yu wrote:
[...]
>> +
>> +int shstk_setup(void)
>> +{
>> + struct thread_shstk *shstk = &current->thread.shstk;
>> + unsigned long addr, size;
>> + int err;
>> +
>> + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
>> + return 0;
>> +
>> + size = round_up(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
>> + addr = alloc_shstk(size);
>> + if (IS_ERR_VALUE(addr))
>> + return PTR_ERR((void *)addr);
>> +
>> + start_update_msrs();
>
> You're setting CET_U with the MSR writes below. Why do you need to do
> XRSTOR here? To zero out PL[012]_SSP?
>
> If so, you can WRMSR those too - no need for a full XRSTOR...
>

Because on context switches the whole xstates are switched together, we
need to make sure all are in registers.

>> + err = wrmsrl_safe(MSR_IA32_PL3_SSP, addr + size);
>> + if (!err)
>> + wrmsrl_safe(MSR_IA32_U_CET, CET_SHSTK_EN);
>> + end_update_msrs();
>> +
>> + if (!err) {
>> + shstk->base = addr;
>> + shstk->size = size;
>> + }
>> +
>> + return err;
>> +}
>